1. Field of the Invention
The present invention relates to a light emitting device for linear light source, in particular a light emitting device for linear light source including a plurality of light emitting elements that are arranged in a line and fixed by die-bonding.
2. Description of Related Art
A linear light source is a light source that casts a light of a desired wavelength such as visible light or ultraviolet light preferentially on a single line, which is used in many fields such as scanner, ink curing and panel pasting. One of conventional light emitting devices for a linear light source was a light emitting device using an electric discharge tube or the like that can be easily formed into a comparatively slender shape.
To address requirements for higher luminous efficiency and reduction in size and weight, recent light emitting devices for linear light source (hereinafter, sometimes simply referred to as “light emitting devices”) include a plurality of light emitting elements such as light emitting diodes arranged in a single line.
For such light emitting devices for linear light source, including a plurality of light emitting elements, what is important is “fine alignment”, i.e. the plurality of light emitting elements are arranged in a straight line with high precision, in order that the light is casted on a single line with high efficiency and/or high precision.
An example of good cell alignment methods known in the art can be found in JP 2008-117900A. It discloses a method including the steps of forming low-wettability areas around die pads on which the light emitting elements will be mounted, wherein the low wettability areas have a lower wettability to a die bond for fixing the light emitting elements onto the die pads than a metal film that forms the die pad, and arranging the light emitting elements by self-alignment.
A light emitting device 500 as illustrated in FIG. 8 is an example of light emitting devices known in the art that are manufactured with high precision by means of self-alignment.
In the light emitting device 500, a metal film 503 formed on a substrate 1 includes low-wettability areas 509 that are placed adjacent to four side edges (four sides of quadrilateral outlines) of quadrilateral light emitting elements 7 fixed at predetermined positions by die-bonding.
That is, because each light emitting element 7 is accompanied with the low-wettability areas in the neighborhood of each side of its quadrilateral outline, the light emitting elements 7 are arranged such that the centers of the respective quadrilateral outlines are positioned on a line A1 of FIG. 8 and either pair of opposing sides of each quadrilateral outline is positioned parallel to the line A1, which results in a desired alignment with high precision.
By providing an optical element as necessary, such as a cylindrical convex lens extending over the plurality of aligned light emitting elements 7, the light emitting device 500 serves as a linear light source.
However, the conventional light emitting device 500 has a problem that the light intensity on the line A1, on which the light emitting elements 7 are arranged (aligned), decreases periodically along the line A1 to a great degree.
As can be seen in FIG. 8, in the conventional light emitting device 500, the line A1 passes through some of the low-wettability areas 509 that are positioned adjacent to a pair of opposing side edges of each light emitting element 7. This makes it impossible to narrow down the distance between the light emitting elements 7 along the line A1. That is, in the light emitting element 500, the distance between the light emitting elements 7 along the line A1 is longer than twice the width of the low-wettability areas 500 (longer than the space enough to place two low-wettability areas 509). As a result, the light intensity along the line A1 decreases at the areas where no light emitting element 7 is laid. Further, in many cases, the low-wettability areas 509 have considerably lower light reflectance than the metal film 503. The low-wettability areas 509 are often made of non-metal material. For example, they are exposed areas of the substrate 1 where the metal film 503 is not formed. Such material often has considerably lower reflectance than metal.
As a result, the light intensity along the line A1 is further decreased at the areas where the low-wettability areas 509 are laid.
As can be seen from the above, the light intensity measured along the line A1 is periodically decreased corresponding to the areas where the light emitting elements 7 are not mounted.